The present invention relates to a polymer electrolyte fuel cell to be used for a portable power source, an electric vehicle, a cogeneration system, or the like, and also relates to a method of manufacturing the same and inspection method therefor.
In a hydrogen ion conductive polymer electrolyte fuel cell, a fuel gas containing hydrogen electrochemically reacts with an oxidant gas containing oxygen, thereby generating electric power and thermal energy simultaneously. To construct such fuel cell, a catalyst layer having, as a main ingredient, a carbon powder carrying a platinum group metal catalyst and having a catalyst function is formed on both major surfaces of a hydrogen ion conductive polymer membrane, which selectively transports hydrogen ions. Next, a porous supporting body which is made, e.g., of a carbon cloth, a carbon non-woven fabric, or a carbon paper, and which has both fuel gas permeability and electronic conductivity is provided as a gas diffusion layer on the outside surface of each catalyst layer. The combination of the gas diffusion layer and the catalyst layer constitutes an electrode. The electrode for the fuel gas is called the anode, while the electrode for the oxidant gas is called the oxidant electrode or cathode.
Next, in order to prevent the supplied fuel gas from leaking outside and from mixing with the oxidant gas, gas sealing members or gaskets, which sandwich the polymer electrolyte membrane therebetween, are placed around the electrodes. The sealing members or gaskets are preliminarily assembled integrally with the electrodes and the polymer electrolyte membrane. The combination of the electrodes and the polymer electrolyte membrane sandwiched between the electrodes is called the electrolyte membrane-electrode assembly (MEA).
A hot pressing process at about 100 to 150° C. is usually used for bonding the catalyst layer and the gas diffusion layer, thereby increasing the handleability in MEA assembling and also increasing the intimate contact between the catalyst layer and the gas diffusion layer. The intimate contact increases the gas reactivity and decreases the contact resistance between the layers. Outside the MEA, electrically conductive separator plates are placed for mechanically fixing the MEA and for electrically connecting neighboring MEAs in series. Each separator plate is provided, on each surface thereof for contact with the MEA, with a gas flow channel for supplying a reactive gas to the electrode and for carrying away generated gas and excess gas to outside. The gas flow channels can be provided separately from the separator plates, but generally the separator plates are provided on the surfaces thereof with gas communication grooves as the gas flow channels.
A pair of neighboring electrically conductive separator plates having an MEA sandwiched therebetween constitutes a unit cell. A fuel cell comprises a cell stack having stacked unit cells.
In order to supply a fuel gas to the gas flow channel, it is necessary to furcate a pipe for supplying the fuel gas to pipe branches corresponding to the number of the assembled separator plates, and to prepare a piping jig for directly connecting the pipe branches to the gas flow channels of the respective separator plates. This jig is called a manifold. The type of manifold as described above, in which the supply pipe is directly connected to the gas flow channels, is called an outer manifold. Another type of manifold, which has a simpler structure, is called an inner manifold. The inner manifold is of such a type that separator plates having gas flow channels are provided with apertures or through-holes, and the inlets and outlets of the gas flow channels are connected to the apertures, through which the fuel gases are supplied or exhausted.
A fuel cell generates heat during its operation. Therefore, in order to maintain the fuel cell at an appropriate temperature condition, it is necessary to cool the fuel cell, e.g., by cooling water. In a cell stack of a fuel cell, a cooling unit is provided to be inserted between neighboring separator plates for every 1 to 3 unit cells. An often employed manner is to bond two separator plates, each having a cooling water flow channel on one surface thereof, such that the surfaces thereof each have the cooling water flow channel facing each other, thereby forming a cooling unit.
Such MEAs, separator plates and cooling units are alternately stacked to a stack of about 10 to 200 cells, thereby forming a cell stack. The cell stack is sandwiched by a pair of current collecting plates, insulating plates and end plates in this order, and is then fixed by tightening bolts provided at the end plates to tighten the cell stack. This is a general structure of a stack type polymer electrolyte fuel cell.
In a conventional fuel cell, a cell stack thereof is usually tightened by a tightening pressure of about 10 to 20 kgf/cm2 for the purpose of decreasing the contact resistances among the polymer electrolyte membrane, the electrodes and the separator plates, and for ensuring the properties of gas sealing with the gas sealing members or gaskets. For this reason, the end plates are generally composed of mechanically strong metal plates, and the tightening bolts are combined with springs or washers for applying a sufficient tightening pressure to the cell stack. Further, stainless steel, which has high resistance to corrosion, is usually used as a material for the end plates, because the end plates partially contact humidified gases and cooling water. Further, with respect to the current collecting plates, metal plates having greater electrical conductivity than carbon plates are usually used, and in some cases are subjected to surface treatment for decreasing the contact resistance. Further, since the end plates electrically contact each other through tightening bolts, an insulating plate is inserted between the current collecting plate and the end plate at each end of the cell stack.
On the other hand, the electrically conductive separator plates to be used for such polymer electrolyte fuel cells need to have a high electrical conductivity, high gas tightness against fuel gases, and high resistance to corrosion, i.e., high resistance to acid, during oxidizing and reducing reactions between hydrogen and oxygen. For these reasons, the separator plates are usually made of gas impermeable and dense carbon plates, which are provided with gas flow channels by cutting. Alternatively, the separator plates are often made by hot pressing a mixture of a thermosetting resin and a graphite powder placed in a press mold having a convex pattern corresponding to the gas follow channels.
In place of using such carbon materials, it has also been attempted to use metal plates, such as stainless steel, for the separator plates. Separator plates using metal plates are likely to become corroded or dissolved during long term use, because the separator plates are exposed to an acid atmosphere at a relatively high temperature. The corroded portion of the metal plate increases the electrical resistance, resulting in a decrease of the fuel cell output. Furthermore, when the metal plate becomes dissolved, the dissolved metal ions diffuse into the polymer electrolyte membrane, and are exchanged with ion exchange sites in the polymer electrolyte membrane, so that consequently the ionic conductivity of the polymer electrolyte membrane itself decreases. For avoiding such deterioration, each metal plate is usually subjected to a noble metal plating, thereby forming a noble metal layer having a sufficient thickness on the surface thereof.